Tube Bass Amp-Whatdaya Think

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Twangmaster
Junior tube assistant
Posts: 21
(5/23/05 8:41 am)
Reply  Tube Bass Amp-Whatdaya Think
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 I have a project idea that I would like to build for a graduation present for this nice kid at my church. He is an awesome bass player and also I would like to introduce the next generation to tube amps.

What is a good classic tube bass circuit I could look at?

Here is what I have to work with:

xs.to/xs.php?h=xs30&d=052...e2rev1.JPG

Here is what I'm thinking, change the rectifier to diodes, run 4 KT66 or 2 KT88's if the tranny would handle it.
I will keep the preamp octal and run a 6SJ7 pentode pre and a 6SL7 as a phase inverter.Also add negative feedback to keep things clean. I am not really familiar with the 6j5 tubes in ther now. I would get rid of the tremelo circuit in there also.

For the tone stack, I would like to do a baxandall with treb,mid, bass. Not to sure about this but Machless used a cathode follower to dive this?

Since there is not a bias voltage tap on the PT secondary, what could I do there?

Thanks for any help and or ideas.
Denny

PS: i Deleted some of the tremelo circuit and connectors to clean up the schematic a bit. Here is the original:
xs.to/xs.php?h=xs30&d=052...-type2.JPG

Edited by: Twangmaster at: 5/23/05 9:28 am
 
j allen shaw
I only work on Fender's
Posts: 693
(5/23/05 8:45 am)
Reply | Edit
  Re: Bias taps
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 As for the bias tap, you can tap it right off of the HT....no separate tap is needed.
Use this as a guide.....bias methods without taps

Jeff
 
j allen shaw
I only work on Fender's
Posts: 694
(5/23/05 8:52 am)
Reply | Edit
  Re: Rectos
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 Also, since you have (2) recto sockets available, I think 2 x GZ34's would be cool!
Wouldn't that give close to diode stiffness, but keep some of the tube feel & "mojo"?

$.02
Jeff
 
Twangmaster
Junior tube assistant
Posts: 22
(5/23/05 11:05 am)
Reply  Re: Rectos
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 Thanks for the link J. Am I reading it correctly that the top example is the best one to use?

Yeah, 2 GZ-34's would be cool, or maybe thats hot...it could double as a space heater. I thought maybe with SS rectos that I could get away with running 2 KT88's for possibly more power? I'm just kind of testing the waters at this point.  
 
HotBluePlates
I only work on Fender's
Posts: 1183
(5/23/05 11:55 am)
Reply  bass amp
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 I'd feel like 2 GZ34's would be overkill. They are so close to the feel of ss diodes already that with 2 of them, you might as well be using the diodes. I just think it would lose any tube rectifier effect at that point and be expensive.

I'd personally go for the KT88 approach or use 6550's. 4 output tubes is expensive no matter what you use, and max clean power is the goal with most bass amps. Using fewer tubes while getting the power is probably the way to go. Both 6550's and KT88's will deliver lots of power and solid lows.

Another way to go would be the Ampeg B-15. It uses 2 6L6's for output, as well as a tube rectifier. Not as loud and punchy as the other options above, but a good sounding bass amp.

But again, I'd probably stick to the 6550's and KT88's. 6550's would be less expensive, whether you're looking at new or NOS.
 
yellatele88
Senior tube assistant
Posts: 144
(5/23/05 11:57 am)
Reply  leslie
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 Twang,

From what I've seen with Leslies in my experience, and from looking at the schematics I have for the 31H - most of them received HT voltage from the organ console to power the PT's primary.

I'm not sure if you are planning to use a donor Leslie for the project, or if you are simply looking at the amp for ideas. I just wanted to let you know what you may run into along the way if you start from a Leslie amp.

Good luck,

Bob
 
Twangmaster
Junior tube assistant
Posts: 23
(5/23/05 12:19 pm)
Reply  Tube Bass Amp
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 Hotblueplates: Do you know if I could get the same or more power from 2 KT88's than from 4 6L6 or KT-66's?

Yellatele88: Not sure what HT Voltages mean in your post. Did you check out the link's for the schematic I posted? Looks like everything that I would need is there.
 
yellatele88
Senior tube assistant
Posts: 145
(5/23/05 12:33 pm)
Reply  Re: Tube Bass Amp
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 Sorry, I didn't phrase my last post correctly.

If you go to: captain-foldback.com/Lesl...matics.htm

there are 2 revisions of the Leslie 31H. In one of them, the main power (ie: line voltage) to the power transformer actually is fed via pin 3 & 4 on the 6-pin console plug.

Also there is a 300v B+ that feeds in from the organ console to feed the relays.

check out hamtech.org/leslie_cable_guide.htm

That page has what you need to decipher the 6pin input so that you can do some rewiring to get the amp to work for another instrument.

Good luck

 
 
Twangmaster
Junior tube assistant
Posts: 24
(5/23/05 4:11 pm)
Reply  Re: Tube Bass Amp
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 Yellatele88,

Yeah I see what you are talking about. The plug sockets would get disconnected and a new grounded power cord installed. Also the tremolo circuit with the relay would be pulled. I could then use the tremelo tube socket for an octal preamp tube.

I think that both of the other 2 6J5 tubes are part of the phase inverter circuit!?! Someone please correct me if I'm wrong. So it must be getting the preamplified signal from the organ.
 
6G6
I only work on tube amps
Posts: 417
(5/23/05 4:55 pm)
Reply  Re: Tube Bass Amp
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 6SL7, 6SJ7 and a pair of KT 88s sounds like a Sunn S200, just off the top of my head.
I'm still looking for a schematic for that.
 
6G6
I only work on tube amps
Posts: 418
(5/23/05 5:18 pm)
Reply  Re: Tube Bass Amp
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 My bad. The weird tube 'key player' in the Sunn is a 6AN8.
www.dangpow.com/~sunn/schematic/220.gif
I must have been thinking Ampeg.
The Sunn would be a good choice or look at Ampeg B15, B25, etc.
The Baxendahl tone stack doesn't have a mid control, but it doesn't need it either.
Find Duncanamps tone stack simulator for a good graphic explaination of how that works.
 
GroundhogKen
Moderator
Posts: 2780
(5/23/05 5:25 pm)
Reply  Re: Tube Bass Amp
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 There is no phase inverter. The 31H gets a differential signal from the organ preamp.

I vote you sell the thing to someone who needs a 31H Leslie amp. Do you have a 31H cabinet?

I try to discourage people from modifying, chopping or molesting any Hammond or Leslie tube instrument.

Ken

 
yellatele88
Senior tube assistant
Posts: 146
(5/23/05 9:46 pm)
Reply  Re: Tube Bass Amp
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 I agree with you Ken, I literally grew up around Hammonds and Leslies - my uncle has quite an affinity for them... I was probably the only 12 year old who knew who Mike Smokowitz (Founder of Trek mods for Hammonds) was and what his parts were actually for.

Question for Ken - What organ model was the 31h intended for? I am most familiar with the 122 and 147. Although I don't play keys, I have a healthy appreciation for them.


Bob

 
 
Twangmaster
Junior tube assistant
Posts: 25
(5/24/05 6:38 am)
Reply  Re: Tube Bass Amp
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 Groundhogken and Yellatele:
The amp is a pull from a long gone cab. If either of you can use it or know someone who can , let me know soon, otherwise it will become a bass amp.
 
HotBluePlates
I only work on Fender's
Posts: 1185
(5/24/05 7:17 am)
Reply  Re: Tube Bass Amp
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 From GEC's data sheet, the KT88 has a plate dissipation of 42W. Depending on the source, 6550's are listed as having a plate dissipation of 35-40W. You won't get double that with 2 tubes in push-pull, but both tubes tend to stay clean longer than 6L6 type tubes (including the KT66).

So 4 KT66's will be as loud as 2 KT88/6550, but my main reason for suggesting these is that you'll have 2 tubes instead of 4, the amp might be more compact, and it will cost your friend less to retube in the future.
 
Twangmaster
Junior tube assistant
Posts: 26
(5/24/05 7:39 am)
Reply  Re: Tube Bass Amp
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 Hotblue, thats what I was thinking also!
 
 
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